1. Field of the Invention
The present invention relates to a motor for driving magnetic disk and more particularly to a motor for driving magnetic disk adapted to prevent vibrational noise of the driving motor from being transmitted to the magnetic disk device.
2. Description of the Prior Art
The progress in technologies in recent years to provide high performance computers, personal computers in particular, is remarkable. Also remarkable is achievement of extremely high speed operation of the computer. Keeping pace with it, realization of ever higher performance of spindle motors for driving magnetic disk is strongly demanded. While achievement of high performance is demanded, it is also demanded to provide magnetic disks driving motors being simple in structure and manufactured at low cost.
There has been made a proposal to meet such demands and a magnetic disk driving motor realizing it is disclosed in the gazette of Japanese Patent No. 2590334. This magnetic disk driving motor is, in a motor for driving magnetic disk including, as shown in FIG. 4, a fixing frame 1b, a center shaft 47 set up on the fixing frame 1b, a pair of bearings 37 and 38 fitted over the large-diametrical portion 49 of the center shaft 47 with a predetermined distance apart from each other, a rotor frame 12a in a cup form fitted over the pair of bearings 37 and 38, a permanent magnet 45 mounted on the rotor frame 12a, laminated cores 6 constituting armatures 5, and armature coils 21 would around the laminated cores 6, characterized in that the fixing frame 1b is provided with a center axial pipe 2b, the small-diametrical portion 48 of the center shaft 47 is put inside the free end portion of the center axial pipe 2b, the laminated core 6 is fitted over the outer peripheral face of the center axial pipe 2b, a hole 35 is made in the fixing frame 1b, and a lead wire 23 from the armature coil 21 is led out through the hole 35, whereas the rotor frame 12a has a cylindrical wall 43, which is provided with a disk supporting table 15 for mounting a magnetic disk such that the magnetic disk is mounted around the cylindrical wall 43.
Further, referring to FIG. 4, reference numeral 29 denotes a flange, 32 denotes a large-diametrical portion formed at the base portion of the center axial pipe 2b, 33 denotes a medium-diametrical portion contiguous to the large-diametrical portion 32, and 34 denotes a printed board.
In the above described prior art example, the rotor frame 12a borne by the pair of bearings 37 and 38 for rotation is held in a cantilevered manner on the center shaft 47 mounted on the center axial pipe 2b having the armatures 5 fixed thereon. Therefore, though the side wall 43 of the rotor frame 12a is extended so as to cover the outer periphery of the armature 5, it is enabled to make accurate concentric rotation not swaying with respect to the armature 5. Accordingly, the magnetic disk set on the rotor frame 12a can make accurate rotation free from irregularities such as swaying. Further, since the motor is not provided with end plates surrounding the armature 5 on the side of the fixing frame 1b, the number of the components can be decreased and it is made possible to lead out the coil lead wire 23 through the hole 35 made in the fixing frame 1b and, hence, the structure of the whole of the motor can be greatly simplified. Further, since the armature 5 is fixed on the outer peripheral face of the center axial pipe 2b on which the center shaft 47 is mounted, such an effect is stated to be obtained that stress is applied radially and inwardly toward the center axial pipe 2b by the installation of the armature 5 so that the small-diametrical portion 48 of the center shaft 47 is compressed and, thereby, the center shaft 47 is fixed more securely to the center axial pipe 2b.
In the above described prior art example, however, since the laminated core 6 is press-fitted over the periphery of the medium-diametrical portion 33 of the cylindrical center axial pipe 2b rising from the fixing frame 1b, there is such a defect that vibration or noise generated from the armature coil 21 would around the laminated core 6 is transmitted from the interior of the laminated core 6 to the magnetic disk through the center shaft 47 and the rotor frame 12a. Further, when laminated core 6 is press-fitted over the center axial pipe 2b, it sometimes occurs that a large stress is applied to the medium-diametrical portion of the center shaft 47 and, thereby, the same is deformed and the positional accuracy of the rotor frame is deteriorated.
The present invention was made to overcome the difficulties as described above and it is an object of the present invention to provide such a motor for driving a magnetic disk, in a motor for driving a magnetic disk having a stator for generating a revolving magnetic field, exciting magnetic poles formed of a permanent magnet disposed around the stator in confronting relationship therewith, and a rotor frame in a cup form having the exciting magnetic poles on the inner side of its lower portion, rotatably held on a center shaft in a cantilevered manner, and supporting a magnetic disk, that is adapted not to transmit noise generated from the stator to the magnetic disk.